I'll go first: One of my former bosses (partner at the firm at which I was working).
He was a brilliant attorney and manager, well-respected and generally well-liked, although known for a temper that would occasionally rear its head at work. Speaking of work, he worked like a madman, easily 80+ hour weeks of real work, like real actual work, not messing around on the internet or taking long lunches.
During my 2nd year, his appearance started to degrade. He would show up to work disheveled, shirt partially tucked in, his hair tossed and unstyled, reeking of scotch. It wasn't until his work product suffered that the firm really had to confront the issue.
The decline was fast. I have no idea how long he hid it, or for how long his alcoholism was just under the "alarming" threshold. He could have been operating at the boundary between functional and trainwreck for years. Some people hide it well.
Within a few months, he gained 50 pounds, his skin broke out with rosacea, his facial skin began to sag. Nobody quite knew how to handle the situation, although I wasn't privvy to the intimate conversations about his conditions. I heard a rumor that the cleaning crew found two empty bottles of scotch one night (that hadn't been there the previous day).
He could no longer function and was forced to leave the firm. I think he went to rehab, but he never returned to the firm.